Lift-off processes and bi-layer structures are commonly used in the fabrication of surface features on semiconductor substrates/wafers. A parameter of great interest for the bi-layer structures used in the lift-off processes is the undercut, or the distance that the upper layer cantilevers beyond the lower layer. Undercut photoresist structures are used in the semiconductor industry and more particularly in the metal lift-off process as a means of providing a template for metal deposition while minimizing the occurrence of defects such as ‘fencing’. Such defects are the result of the deposited material blanketing both the intended substrate area and the template itself. Removal of the template material then leaves deposited material extending beyond its intended limits. By providing a gap between the substrate and the template by way of an undercut, such defects can be minimized. The amount or width of an undercut should generally be large enough to provide the necessary discontinuity but not so large as to compromise the structural integrity of the template feature.
The bi-layer structure itself often consists of a layer of photoresist over a bottom layer, such as polymethylglutarimide (PMGI), whose dimensions may be tailored through the developer process. Before exposure to developer, both the photoresist and the bottom layer possess the same lateral dimensions. Exposure to photo developer degrades the bottom layer from the outside-in, thereby undercutting the photoresist as a function of time.
The standard method for monitoring undercut (e.g., with widths of less than about 1-2 microns or “um”) involves using a focused ion beam and a scanning electron microscope (FIB/SEM), where undercut parameters such as width are directly obtained from the cross-sectional images of the bi-layer structure. Although the FIB/SEM method for undercut metrology provides a reasonable characterization of the undercut structure, the method is limited due to high capital expenses, service expenses, and operator expenses combined with inherently long processing times associated with the FIB/SEM bi-layer metrology. Further, the FIB/SEM method is a destructive process, thereby requiring the use of sampling to estimate the parameters of the device population. Accordingly, an improved system and method for measuring undercut is needed.